EMI filter capacitors designed for direct body fluid exposure

ABSTRACT

An EMI filter capacitor assembly utilizes biocompatible and non-migratable materials to adapt electronic components for direct body fluid exposure. The assembly includes a capacitor having first and second sets of electrode plates which are constructed of non-migratable biocompatible material. A conductive hermetic terminal of non-migratable and biocompatible material adjacent to the capacitor is conductively coupled to the second set of electrode plates. One or more conductive terminal pins having at least an outer surface of non-migratable and biocompatible material are conductively coupled to the first set of electrode plates, while extending through the hermetic terminal in non-conductive relation. The terminal pins may be in direct contact with the first set of electrode plates, or in contact with a termination surface of conductive connection material. The termination surface is also constructed of non-migratable and biocompatible materials. Layers of glass may be disposed over surfaces of the assembly, including the capacitor.

RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 10/778,954, filedFeb. 12, 2004 (now U.S. Pat. No. 6,985,347), which is acontinuation-in-part of U.S. Pat. No. Application Ser. No. 10/377,086,filed Feb. 27, 2003 (now U.S. Pat. No. 6,765,779), which claims benefitfrom U.S. Provisional Patent Application Ser. No. 60/360,642, filed Feb.28, 2002.

BACKGROUND OF THE INVENTION

This invention relates generally to feedthrough capacitor terminal pinsubassemblies and related methods of construction, particularly of thetype used in implantable medical devices such as cardiac pacemakers(bradycardia devices), cardioverter defibrillators (tachycardia),neuro-stimulators, internal drug pumps, cochlear implants, ventricularassist devices, and other medical implant applications, to decouple andshield undesirable electromagnetic interference (EMI signals) signalsfrom the device. More specifically, this invention relates to materialsand methods of manufacturing monolithic ceramic feedthrough capacitorsso that they can be exposed to body fluid.

It is well known in the art that EMI feedthrough capacitors can beattached to the flanges of human implantable hermetic seals for reliableEMI filter performance. These EMI filters are very important to bypassand attenuate RF signals from undesirable emitters, such as cell phones,microwave ovens and the like.

These devices are generally designed with one or more monolithic ceramicfeedthrough capacitors or monolithic ceramic rectangular chip capacitorsdesigned to be in intimate relation with the hermetic terminal. Ingeneral, monolithic ceramic capacitors are considered to be sensitiveelectronic components and are not manufactured of biocompatiblematerials. Monolithic ceramic capacitors are typically constructed of abarium titinate dielectric into which active and ground electrode platesare interspersed. It is common in the art that the ceramic capacitordielectric be of barium titinate, zirconium titinate, or other highdielectric constant ceramic materials with various dopants added tocontrol its dielectric constant, temperature stability and electricalproperties. Barium titinate in itself is biocompatible; however, theelectrodes and the termination materials are generally notbiocompatible. Typical monolithic ceramic capacitors would include apalladium-silver, or nickel silver electrode system (base metalelectrode). Other electrode systems are possible, including ternary,which is a high fire system consisting of an alloy of gold, platinum andpalladium.

Typical capacitor termination materials are applied in two ways. Thefirst system involves a glass frit, which is loaded with metallicparticles along with a binder and vehicle system to make a paste. Thispaste is then applied to the capacitor and fired into place. Theconductive particles make contact to the exposed electrode plates andplace them in parallel. A properly formed capacitor termination is ahighly conductive surface to which one can make electrical connectionsthrough soldering or other methods. Typical materials used for thisglass frit are a silver or copper loaded glass frit or a palladiumsilver or platinum silver composition. Silver is relatively inexpensiveand highly conductive and is also available in a wide variety of flakesand spherical shapes. Accordingly, it is well known in the art to builda monolithic ceramic capacitor using such termination material.

The second methodology involves plating of the termination. There are anumber of plating methods currently used in the art, including a barrierplating technique which consists of plating down nickel and then variousmaterials on top of the nickel to promote solderability. The nickel actsas a barrier layer and prevents leaching off of the capacitor. Forexample, if tin or copper were plated on top of the nickel, the tin orcopper would readily wet with solder and the nickel would form a layerresistant to leaching or removal from the capacitor. Therefore, innearly all of the prior art devices the monolithic ceramic capacitor isplaced on the inside of the implantable medical device. In other words,this places the sensitive monolithic ceramic capacitor away from thebody fluid so that it cannot come in contact with the body fluid.Another way of stating this is that a hermetic terminal is used toprevent intrusion of body fluid into the interior of the electronicdevice. Accordingly, all of the electronic circuits, including thesubstrate, circuit boards, battery, computer chips, capacitors andelectromagnetic interference capacitors, are placed in a suitablelocation inside the titanium housing of the implantable medical deviceso that they are protected from body fluids.

However, modern pacemakers and implantable defibrillators tend to bevery small in size and very cramped in terms of space inside the unit.Thus, placing the capacitor on the outside of the housing increases thevolumetric efficiency of the overall design, such as by allowing alarger battery to be inserted in the device housing. In addition, laserwelds used to seal the housing, typically comprised of titanium, willhave a lesser effect on the capacitor.

Recognizing this, U.S. Pat. No. 6,055,455 discloses a monolithic ceramiccapacitor placed on the outside (or the body fluid side) of the hermeticterminal of an implantable medical device. In this patent the concept ofdecoupling the EMI before it gets to the inside of the pacemaker or theimplantable medical device is emphasized. However, it makes nodifference from a filter effectiveness point of view whether thecapacitor is on the inside surface or on the outside surface of thehermetic seal.

Electromagnetic interference consists of a number of modulated carrierfrequencies, for example, the carrier frequency of a typical cellularphone. What is important is that the gap between the feedthroughcapacitor and the hermetic seal be a wave-guide beyond cut off. In otherwords, that gap needs to be small enough so that the wavelength of theelectromagnetic interference will not readily pass through it. As itturns out, after doing wave-guide calculations, this is relatively easyto do for a medical implant application. One reason for this is thehuman body's tendency to reflect and absorb EMI at frequencies of 3 GHzand above. In other words, it really makes no difference whether the EMIfeedthrough capacitor is on the body fluid side or the inside of thehermetic terminal of an implantable medical device. The closely spacedfeedthrough capacitor presents such a small wave-guide that it wouldtake frequencies in excess of 20 GHz to effectively re-radiate aroundthe filter. However, as previously mentioned, at frequencies of 3 GHzand above the human body is so effective in attenuating such signalsthat higher frequencies are really not of importance.

A significant mistake found in the prior art is the notion that addingsome sort of an adjunct sealant over the top of a monolithic ceramicfeedthrough capacitor will allow it to operate in the presence of bodyfluids. Body fluid is an extremely corrosive and conductive medium.There are many dissolved minerals in body fluid, including salt andpotassium, which readily conduct electricity in their ionic state.Polymers and adjunct sealants and conformal coatings on electroniccomponents have a number of weaknesses which include problems withadhesion and also bulk permeability. Simply stated, over a long periodof time moisture can still penetrate through any adjunct non-hermeticsealant and eventually reach the capacitor. In addition, adjunctsealants and coatings have a different thermal coefficient of expansionas compared to the barium titinate ceramic capacitor. Thus, afterexposure to temperature excursions or simply after a long period oftime, the adhesion of the coating to the capacitor surface starts tobreak down. This could allow a thin film of moisture or body fluid to bepresent at the surface of the ceramic capacitor. In fact, any slightseparation of any of the adjunct sealant could cause a small gap ortightly spaced separation into which moisture could easily form. One waythat moisture can form in such a tiny space is through dew pointcondensation. That is, during temperature excursions moisture laden orvapor laden air could enter such a small separation and then deposit outas a thin film of moisture.

One of the most common and severe failures of electronic componentscomes from a process known as metal migration, whisker formation ordendritic growth. A dendrite can form of various migratable materials,including silver, tin, and the like. Another common way of describingthis phenomenon is through tin or silver whiskers. Once these dendritesform across the surface of the capacitor, the capacitor's insulationresistance drops dramatically. This can short out the capacitor, therebyshorting out the entire implantable medical device. The effect couldalso be degraded insulation resistance, which could result in reducedbattery life or in reduced functionality of the output waveform of theimplantable medical device.

FIG. 1 is a cross-sectional view of a prior art unipolar capacitor 10,similar to that described by U.S. Pat. No. 4,424,551, the contents ofwhich are incorporated herein. At first glance it would appear that thecapacitor 12, shown inside the ferrule 14, is well protected againstbody fluid by the sealant 16, such as an epoxy seal. However, in actualpractice there is a mismatch of thermal coefficient of expansion betweenthe polymers and the barium titinate of the ceramic capacitors. Thereare also adhesion problems and difficulties with bulk permeability.Accordingly, across both the top and bottom surfaces of the capacitor 12one can usually see, at high magnification, a small separation 17 isoften present between the sealing material and the capacitor surfaceitself. This would be a separation on the top surface of the capacitor12 and sealing material 16 due to a separation in the bond betweennon-conductive sealing material 16 and the capacitor 12. After aprolonged period of time, moisture can penetrate into either one ofthese spaces. Accordingly, a metal migration or dendrite 18 can formeither on the top or bottom of the capacitor 12. As mentioned above, theformation of this dendrite could lead to either immediate or latentcatastrophic failure of the implanted medical device.

With reference to FIGS. 3–5, a prior art unipolar feedthrough capacitor20 mountable to a hermetic terminal of an implantable medical device,such as a cardiac pacemaker, an implantable cardioverter defibrillator(ICD) a cohlear implant, or the like is shown. Such prior art capacitors20 are typically constructed using a silver-bearing or palladium silverbearing-glass frit for the outside diameter termination surface 22 aswell as the inner diameter surface 24. Connecting material 26 connectsthe capacitor's lead wire 28 to the inside diameter surface 24 of thefeedthrough capacitor 20. The material 26 is typically of asilver-filled conductive polyimide, or a lead or tin bearing solder orthe like. If the capacitor 20 were exposed and placed on the body fluidside of the medical device, a thin film of moisture 30 would be presentacross the surface of the capacitor. This moisture could be present fromdirect immersion in body fluid or from the penetration of any adjunctsealants by body fluids. In the presence of moisture 30, dendrites ormetal migration 32 would form or grow between the areas of oppositepolarity 22 and 24. This dendritic growth or migration can also occurfrom the capacitor's outside diameter metallization material and thematerial used to make the electrical mechanical connection between thecapacitor lead wire 28, and the capacitor's inside diameter 24. Even ifthe capacitor's outside diameter termination material 22 was ofbiocompatible material, (which is not typical in the prior art), theconnection material 26 which forms the electromechanical connection fromthe capacitor outside diameter 22 to a ferrule 34, could still beproblematic. That is due to the fact that the connecting material 26 istypically a silver-filled conductive thermosetting polymer, such as aconductive polyimide or the like.

Thus, in the presence of moisture and a voltage bias, the silver is freeto migrate and form dendrites 32 as shown in FIGS. 3 and 5. Of coursethose skilled in the art will realize that the formation of thesedendrites 32 is highly undesirable because they are conductive and tendto lower the insulation resistance or short out the capacitor 20. Thisis particularly problematic in a low voltage pacemaker application. Incardiac pacemaker applications, the formation of the silver, tin orother dendrites 32 would preclude the proper operation of the implantedmedical device. Another undesirable effect of the formation of thesedendrites 32 is that they would tend to conduct current and therebydissipate power unnecessarily, leading to premature battery failure ofthe implanted medical device. Premature battery failure is highlyundesirable and leads to unwanted surgery and increased expense, usuallythe replacement of the entire implantable medical device.

With reference now to FIGS. 6 and 7, a surface mounted quadpolarcapacitor 36 is illustrated, such as that described in U.S. Pat. No.5,333,095, the contents of which are incorporated herein. As can be seenfrom the illustration, dendrites 38 or 38′ can form between any pointsof opposite polarity as long as there is migratable material as well asa migratable medium. As previously mentioned, migratable mediums includethin films of moisture, solvents or the like. Accordingly, anotherproblem can arise during cleaning or washing of the capacitor 36. Anyentrapped cleaning solvents, such as alcohol, water or degreasers alongwith a bias voltage can allow for the migration of the metallicmigratable materials. It will be appreciated by those skilled in the artthat not only can the dendrites 38 form between lead wires of oppositepolarity 40, but also at 38′ between two lead wires of the same polarityand an adjacent ground at the capacitor outside diameter metallization42. Both conditions are highly undesirable in that the dendrite 38formation could short out or reduce the insulation resistance betweenthe two lead wires thereby degrading any biological signal sensing thatthey may perform. The term “short out” does not necessarily imply thatthe dendrite 38 will form a zero ohm connection because the resistanceof the dendrite, metal migration or whisker depends upon a number offactors including the thickness density and length of the dendrite 38 or38′ that is formed. Dendrites do not form a continuous sheet, but ratherare discontinuous. Time lapse photography has shown that dendrites formside branches similar to a tree with many leaves. Accordingly, whatresults is a matrix of silver conductive particles that have manystrange geometric shapes. Accordingly, the resistivity of such astructure is highly variable, ranging from several thousand ohms down toa very few ohms.

With reference to FIG. 8, an in-line quadpolar capacitor 44 isillustrated wherein the outside or ground termination 46 is in twolocalized areas. Such localization minimizes the opportunity fordendrites to form. However, when the electrical connection is madebetween the termination material 46 and the conductive ferrule material48 using a connective material 50 which is comprised of migratablematerial, typically a silver-filled solder or conductive thermal-settingpolymer such as a conductive polyimide or the like, the formation ofdendrites 52 or 52′ is possible in the presence of moisture. A dendrite52 could form between the capacitor conductive metallization 46 and leadwire 54 or a dendrite 52′ could form between lead wires 54, asillustrated.

With reference to all of the illustrated prior art, when the capacitoris installed in the housing of an implantable medical device and thecapacitor is oriented toward the inside, such dendrites typically do notform. This is because the inside of the implantable device ishermetically sealed. This prevents intrusion of body fluids or othermoisture. In addition, the active implantable medical device istypically thoroughly cleaned and then baked dry prior to assembly. Thedevice is then laser welded shut. Prior to final sealing, the interiorof the implantable medical device is evacuated at high vacuum and thenback-filled with dry nitrogen. In other words, the ceramic capacitors ofthe prior art are never really exposed to moisture throughout theirdesign life. Accordingly, the dendrites 52 in FIG. 8 do not have achance to form when the capacitor is oriented to the inside of aproperly constructed active implantable medical device.

FIG. 10 illustrates a prior art internally grounded bipolar feedthroughfilter capacitor 56, such as that disclosed in U.S. Pat. No. 5,905,627the contents of which are incorporated herein by reference. Even thoughthe capacitor 56 has no outside diameter or outside perimetermetallization, a dendrite 58′ can still form if a moisture film andvoltage bias form between the lead wires 60 and 66 or a dendrite 58 canform between a lead wire 60′ and a conductive ferrule 62. In this case,the conductive ferrule 62 has been greatly simplified and shown as arectangular plate. In the art, these ferrules 62 take on a variety ofsizes and shapes, including H-flanges to capture the mating halves of animplantable medical device housing. As shown, the dendrite 58 has formedall the way from the conductive material to the ferrule 62 used to makethe connection between the capacitor lead wire 60 and the capacitorinside diameter 64, which would typically be a conductive polyimidesolder or the like. In an internally grounded feedthrough capacitor 56,there is always a grounded lead wire 66 which is connected to thecapacitor's internal electrode plate set 68, illustrated in FIG. 12. Itis also possible, or even likely, to form a dendrite 58′ between thislead wire and any adjacent lead of opposite polarity. Such a dendrite58′ would short out the lead wire 60 to the grounded lead wire 66. Thisis why coating such leads, which may be formed of noble metal material,with migratable metals or materials such as tin-lead combinations, isproblematic. Thus, it will be readily apparent by those skilled in theart that dendrites can form and migrate over any migratable conductivematerial, such as silver-filled conductive thermal-setting connectivematerial which is often used to connect lead wires 60 and 66 to theinside diameter metallization 64 of the feedthrough capacitor orconductively connect the outside of the capacitor to the ferrule 62.

It should be noted that for a dendrite to form, the migratable materialneed not be present on both sides. In other words, a migratable materialis not necessarily both the cathode and the anode. There are nomaterials in titanium that would migrate, however, silver particles fromconductive silver bearing glass frit fired onto the capacitor is capableof migrating in the presence of a voltage bias and a moisture film. Itis also possible that a dendrite material form directly between theinside diameter metallizations 64 from the ground feedthrough hole andone or more of the active insulated feedthrough capacitor wires.

Detecting the presence of these dendrites can sometimes be veryconfusing for the test technician. This is because the dendrites mostreadily form in a high-impendence, low voltage circuit where a moisturefilm is present along with migratable materials. The dendrite, metalmigration or metal whisker is typically very lacy, thin and of lowcross-sectional area. Accordingly, this material can act like a fuse andopen up if a high voltage or a low impedance voltage or current sourceis applied. Accordingly, when dendrites are present, they are sometimesinadvertently blown open by routine electrical testing either by themanufacturer or by the customer's receiving inspection department. Aconcern is that after years of field use, if the dendrite were toreform, this could slowly degrade the battery life of the medical devicethrough decreased insulation resistance or degrade the device's abilityto sense very low level biological signals. These are yet again reasonswhy it has been common in the prior art to always place the ceramicfeedthrough capacitor toward the inside where it is protected from bodyfluids.

FIG. 13 shows a prior art integrated chip capacitor 70, such as thatdescribed in U.S. Pat. Nos. 5,959,829 and 5,973,906, the contents ofwhich are incorporated herein. These chip capacitors 70 come in avariety of sizes and shapes and are used to decouple electromagneticinterference from the lead wires 72 of an implantable medical device tothe metallic ferrule 74. As illustrated, capacitor 70 has integratedfour rectangular chip style capacitors into a single monolithic package.Each of these chip capacitors makes a connection to the lead wire 72 anddecouples EMI to the metallic ferrule 74. Since prior art chipcapacitors are constructed of the same materials as are typical in theentire capacitor industry, it is likely that a dendrite 76 will form ifmoisture or solvents are present. Such dendrites 76 can form between themigratable connective materials used to connect the capacitormetallization 78 to the lead wire 72 and the ferrule 74, or between thelead wires 72 (not shown).

It is a common misconception that it takes many months or years formetal migration or dendrites to form. Actually, the dendrite itself hasbeen observed to form very quickly so long as a migratable material, amoisture or solvent film, and a suitable bias voltage is present. Oncethese three factors come together, it can be only a matter of seconds orminutes for the dendrite itself to actually form. As previouslymentioned, dendrites can also form from lead wires to the conductivematerials used to connect the capacitor's ground termination to theconductive ferrule. This is the case even if the ferrule is of anon-migratable material such as titanium or a noble metal, such as goldor the like, provided that the connective material is of a migratablematerial such as silver, tin, or other known migratable metals. As canbeen seen, there are many ways for such dendrites to form.Notwithstanding U.S. Pat. No. 6,055,455, the inventors are not aware ofa single instance in an implantable medical device where the capacitorhas been placed on the outside and exposed to body fluid. Instead, ithas been standard practice in the medical implant industry that allelectronic components be protected inside the hermetically sealedenclosure, which is typically vacuum evacuated and back filled with aninert gas such as nitrogen or the like to ensure a very dry atmosphere,and prohibit contact with body fluids. Of course, in such a dryatmosphere, one of the three essential ingredients for metal migrationor dendrite formation is removed and such dendrites do not form.

Metal migration, whiskers and dendrite formation does not only occur ofthe surfaces of ceramic feedthrough and chip capacitors. Said dendritescan also form inside the capacitor along microfractures, cracks, or knitline defects (slight separations in the capacitor electrode laminationboundary). Internal metal migration within a ceramic capacitor can havethe same catastrophic effects as surface migration. That is, theinsulation resistance of the capacitor can be severely reduced includingthe shorting out of the capacitor completely.

The ceramic feedthrough capacitor which acts as an EMI filter is poiseddirectly at the point of ingress and egress of the lead wires betweenthe implantable medical device and body tissue. For example, in acardiac pacemaker, the feedthrough capacitor is placed at the pointwhere lead wires from the heart enter into the pacemaker itself.Accordingly, any short circuiting or lowering of insulation resistanceof the ceramic feedthrough capacitor precludes or shorts out the properoperation of the pacemaker itself. This can be very dangerous or evenlife threatening to a pacemaker-dependent patient whose heart depends oneach pulse from a pacemaker so that it itself will beat. There arenumerous instances in the literature wherein cardiac pacemakers,implantable defibrillators and neurostimulators have been shown toadversely react in the presence of an emitter such as a cell phone orretail store security gate (electronic article surveillance system).Pacemaker potential responses to EMI include sensing (pacemakerinhibition), noise reversion to asynchronous spacing, tracking for dualchamber devices, in rate adaptive devices the rate changes withinprogrammed rate limits, activation of the lead switch, ICD undersensing,asynchronous pacing, or microprocessor reset. In an implantablecardioverter defibrillator (ICD), potential responses to EMI can includeall of the responses for a pacemaker in that ICDs often include apacemaker function. In addition, ICDs may also respond to EMI byover-sensing that manifests itself as either inhibition or aninappropriate delivery of therapy. An inappropriate delivery of therapymeans that a fully alert and cognizant patient would receive a highvoltage shock. Delivery of such a high voltage can injure the patient byliterally throwing him off his feet (such a case has been documentedwith the male patient breaking his arm). In addition, ICDs can respondto EMI by tracking, undersending an arrhythmia, or electrical currentdirectly induced in the lead system that can trigger a dangerous cardiacarrhythmia. Accordingly, proper operation of the EMI filter is criticalto protect the implantable medical device from not exhibiting any of thepossible aforementioned malfunctions. Formation of dendrites canseriously degrade the proper operation of the pacemaker and/or make thefilter ineffective at performing its proper function.

For example, with reference to FIGS. 15–17, a cross-sectional view of aprior art unipolar feedthrough capacitor assembly 79 is shown similar tothat described in U.S. Pat. Nos. 4,424,551; 4,152,540; 4,352,951 andothers. Monolithic ceramic capacitors have a relatively low thermalcoefficient of expansion compared to metals. Ceramic capacitors are verystrong in compression, but very weak in tension. This is typical of mostbrittle materials. Accordingly, it is very easy to introduce crackswithin the ceramic capacitor structure if the capacitor is subjected toexcessive stresses. The ceramic capacitor assembly shown in FIG. 15 hasthe ceramic capacitor embedded within a metallic ferrule 80. For a humanimplant application, this metallic ferrule 80 would typically be made oftitanium and could have a variety of shapes and flanges. The connectionfrom the inside diameter of the ferrule 80 to the outside diametermetallization 82 of the feedthrough capacitor is shown as material 84.Material 84 is typically a thermal-setting conductive adhesive, such asa silver-filled conductive polyimide, epoxy or the like. The entireassembly shown in FIG. 15 is designed to be installed into a pacemaker,ICD or the like by laser welding directly into the titanium can of theimplantable device. Accordingly, the ferrule 80 is rapidly heated andtends to expand. The relatively cooler ceramic capacitor 79 does notexpand nearly at the same rate. Accordingly, a variety of cracks can beintroduced into the ceramic capacitor. These cracks can be axial, radialor cover sheet type features.

For purposes of example, as shown in FIG. 17, a crack 86 has propagatedacross the corner of the ceramic capacitor 87. Additionally, the crack86 has contacted plates 88 and 90 of opposite polarity. In other words,the crack 86 has propagated through the main body of the ceramicdielectric 92 between a ground electrode plate 88 and the lower activeelectrode plate 90. This in and of itself does not present an immediateelectrical defect. The reason for this is that as long as the crack 86itself does not contain metallic particles, the two electrodes 88 and 90are not shorted out. However, it is quite possible for these cracks topropagate to the outside diameter or top surface of the capacitor 87.Long-term exposure to body fluid in combination with the bulkpermeability of the surrounding polymers can lead to the presence of amoisture thin film that lines the inside of this crack 86. FIG. 17Ashows a silver dendrite 86′ that has formed by metal migration throughthe cracks. The reason for the formation of the dendrite has to do withthe intrinsic materials that are typically used in the prior artelectrodes and capacitor terminations themselves. Ceramic capacitors aretypically made with nickel, silver or palladium silver electrodes. Theseare low cost electrode systems that are found in many ceramic capacitorstoday. They are formed within the solid monolithic ceramic by firing orsintering at a relatively low temperature (around 1100° C.). Aspreviously mentioned, an internal dendrite is a highly undesirablesituation to occur because this shorts out the ceramic capacitor. Suchshorting or reduced insulation resistance of the ceramic capacitor notonly degrades its effectiveness as an EMI filter, it also can cause thecatastrophic failure of the entire implantable medical device. Asmentioned, this can be life threatening, for example, in the case of apacemaker-dependant patient. The dendrite can be low enough inresistance to short out the pacemaker output pulse. In this case, thepatient's heart would simply stop beating, which would quickly lead todeath.

Accordingly, there is a need for a feedthrough filter capacitor whichcan be disposed on the body fluid side of an implantable device toprovide additional space for an enlarged battery, a smaller implantabledevice, etc., while being immune to dendritic growth. The presentinvention fulfills these needs and provides other related advantages.

SUMMARY OF THE INVENTION

As noted above, three ingredients are needed for such catastrophic orlatent metal migration defects to occur: 1) the presence of a migratablematerial, which may include silver, tin, and many other materials; 2) amigratable medium, such as a thin film of moisture or solvent; and 3) anactivation energy such as an applied voltage. On the outside of acardiac pacemaker which is implanted in body fluid and has a smalloutput voltage, two of these three elements are always present. That is,the migratable medium and the activation energy. Activation energy ispresent in most active implantable medical devices, including cardiacpacemakers, implantable cardioverter defibrillators, neuro-stimulators,cochlear implants, and the like.

For a feedthrough capacitor mounted on the body fluid side, anotherchallenge is the inner connection between the ceramic feedthroughcapacitor or chip capacitor and the lead wires and also the ferrule orthe hermetic terminal. In an electromagnetic interference attenuationapplication, it is important that the capacitor active electrodes beconnected to each lead wire and its ground electrodes connected to theground. As defined herein, ground is the potential of the overallelectromagnetic shield, which in the case of a pacemaker is usuallytitanium, but may be of a titanium alloy, stainless steel, tantalum,ceramics, niobium, or the equivalent. The ferrule is typically laserwelded to this overall titanium can or housing. Therefore the titaniumcan or housing forms an equipotential surface to which EMI can bebypassed by the feedthrough or chip capacitor.

The use of solders to make the electrical connection between thecapacitor and its lead wires or the capacitor and the ferrule, isgenerally ruled out. The reason for this is that most solders containeither lead or tin, both of which are not biocompatible. The problemswith lead are obvious from all of the literature regarding leadpoisoning in the human body. Tin is ruled out because it is notbiocompatible plus it will readily form whiskers or dendrites. Evenexotic gold alloy solders still usually contain a percentage of tin orlead, which rules them out for similar reasons. The trouble with otherconducting materials such as a conductive polyimide or conductive epoxyis that the polymer is loaded with a silver powder, such as a silverflake or a silver sphere, which is not tightly bound up chemically andis free to migrate. This is also true of the prior art ceramic capacitormetallization materials. For example, a silver bearing glass frit whichis fired onto the capacitor will readily form a dendrite. Accordingly,it is a novel feature of the present invention that the thirdingredient, namely, migratable materials exposed to the migratablemedium, be removed. Thus, a preferred embodiment of the presentinvention resides in an EMI filter capacitor assembly adapted for directbody fluid exposure by being constructed of biocompatible andnon-migratable materials, particularly in locations where body fluidexposure occurs. Alternatively, or in addition, the filter capacitorincludes a protective barrier, preferably glass, which prevents the bodyfluid from contacting such conductive and critical portions of thecapacitor assembly.

Thus, in general, the EMI filter capacitor assembly which is adapted fordirect body fluid exposure comprises a capacitor having first and secondsets of electrode plates which comprise a non-migratable andbiocompatible material. A conductive hermetic terminal comprises anon-migratable and biocompatible material and is adjacent to thecapacitor so as to be in conductive relationship to the second set ofelectrode plates. A conductive terminal pin, having an outer surfacecomprising a non-migratable and biocompatible material at least whereexposed to body fluid, is in conductive relationship with the first setof electrode plates and extends through the hermetic terminal innon-conductive relation.

The first and second sets of electrode plates and the outer surface ofthe terminal pin are typically comprised of a noble metal or a noblemetal composition. For example, these structures may be comprised ofgold, tantalum, niobium, platinum, a gold-based alloy or aplatinum-based alloy. The hermetic terminal, usually in the form of aferrule, comprises a material selected from titanium, a titanium alloy,stainless steel, tantalum, a tantalum alloy, niobium, a niobium alloy,gold, a gold alloy, platinum, and a platinum alloy. Such biocompatibleand non-migratable materials avoid the harmful formation of dendrites,as explained above.

Other biocompatible metals and alloys that can be used for the ferrule,capacitor metallization, capacitor electrode, or capacitor connectionmaterials include all of the metals and alloys of titanium, platinum andplatinum iridium alloys, tantalum, niobium, zirconium, Hafnium, nitinol,Co—Cr—Ni alloys such as MP35N, Havar®, Elgiloy®, stainless steel andgold. There are also a number of conductive metal compounds that can beused including ZrC, ZrN, TiN, NbO, TiC, and TaC.

In several embodiments of the present invention, the capacitor includesa first termination surface comprising a non-migratable andbiocompatible material that conductively couples the first set ofelectrodes and is in conductive relation to the terminal pin. Thecapacitor also includes a second termination surface, which alsocomprises a non-migratable and biocompatible material, that conductivelycouples the second set of electrodes and is in conductive relation tothe hermetic terminal.

Usually, a connection material is used to connect the terminal pin tothe first termination surface, and the hermetic terminal to the secondtermination surface. The conductive connection materials are typicallythermal-setting, brazing, welding or soldering materials. So as to benon-migratable, these materials are selected from the group consistingof: gold, gold alloy, platinum, gold-filled-thermal-setting conductivematerial, platinum-filled-thermal-setting conductive material,gold-bearing glass frit, TiCuSiI, CuSiI, and gold-based braze.

Table 1 below shows a more comprehensive list of polymers that can alsobe filled with any of the biocompatible metals mentioned above. Thislist can include a variety of epoxies and polyimide materials inaddition to polyethylene oxide with ionic additions such as NaCl or anyof the other commonly used implantable polymers including polyurethane,silicone, polyesters, polycarbonate, polyethylene, polyvinyl chloride,polypropylene, methylacrylate, para-xylylene and polypyrrhol. Asmentioned, any of these can be made conductive with a biocompatiblematerial, for example, by adding a particulate filler such as platinumor gold powder. There are other materials that could be used includingpyrolytic carbon and Tra-Duct 2902 conductive adhesive.

An insulator is usually disposed between the capacitor and the ferrule,and a hermetic seal connects the insulator and the ferrule. The hermeticseal is comprised of a non-migratable material, at least where exposedto the body fluid, and is typically selected from the group consistingof: gold, gold alloy, platinum, glass, TiCuSiI, CuSiI, and othergold-base compounds.

In one embodiment of the present invention, the capacitor is amonolithic structure which includes an electrode portion as well as aninsulator portion.

The capacitor may be chip capacitor or a feedthrough capacitor. In theinstance of a feedthrough capacitor, the one or more terminal pins willextend through one or more passageways of the capacitor.

In one feedthrough capacitor embodiment of the present invention, theconductive terminal pin is in direct physical contact with the innerfirst termination surface of the capacitor. In alternative embodiments,there is no first termination surface and instead the one or moreterminal pins are in direct physical contact with the active firstelectrode set. To facilitate this, a portion of the terminal pin mayhave an irregular surface, such as a knurled surface. Similarly, thesecond termination surface may be omitted with the second set of groundelectrodes either by directly contacting the hermetic terminal, orconductively coupled thereto with a biocompatible conductive material,as described above.

In another embodiment, the filter capacitor assembly includes acapacitor having a glass layer disposed on a top surface thereof. Aglass layer may also be disposed on a bottom surface thereof. This glasslayer adds strength to the capacitor, but is also intended to preventbody fluid contact with the first and second termination surfaces aswell as conductive connectors of the capacitor assembly to preventdendrites from forming. As added protection, the capacitor assembly maybe comprised of the non-migratable materials, as described above, aswell as having glass layers disposed thereon.

Other features and advantages of the present invention will becomeapparent from the following more detailed description, taken inconjunction with the accompanying drawings, which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a cross-sectional view of a prior art unipolar capacitorhaving an adhesive seal on a top surface thereof, illustrating a smallseparation formed between a sealing material and a top surface of thecapacitor where dendrites may form;

FIG. 2 is an electrical schematic diagram of the capacitor of FIG. 1;

FIG. 3 is a perspective view of a prior art unipolar feedthroughcapacitor mounted to a hermetic terminal of an implantable medicaldevice, and having dendritic growth thereon;

FIG. 4 is an electrical schematic diagram of the capacitor of FIG. 3;

FIG. 5 is a cross-sectional view taken generally along line 5—5 of FIG.3, illustrating internal components of the capacitor assembly;

FIG. 6 is a perspective view of a prior art surface mounted quadpolarcapacitor, illustrating dendritic growth between lead wires and/or leadwires and ground thereof;

FIG. 7 is an electrical schematic diagram of the capacitor of FIG. 6;

FIG. 8 is a perspective view of a prior art in-line quadpolar capacitor,illustrating dendritic growth between conductive portions thereof;

FIG. 9 is an electrical schematic diagram of the capacitor of FIG. 8;

FIG. 10 is a perspective view of a prior art internally grounded bipolarfeedthrough capacitor having dendritic growth between conductiveportions thereof;

FIG. 11 is an electrical schematic diagram of the capacitor of FIG. 10;

FIG. 12 is a cross-sectional view taken generally along line 12—12 ofFIG. 10, illustrating internal components thereof and dendritic growthformed thereon;

FIG. 13 is a perspective view of a prior art integrated chip capacitorhaving dendritic growth thereon;

FIG. 14 is an electrical schematic diagram of the chip capacitor of FIG.13;

FIG. 15 is a cross-sectional view of a prior art unipolar capacitor,having cracks and dendritic growth between electrodes thereof;

FIG. 16 is an electrical schematic diagram of the capacitor of FIG. 15;

FIG. 17 is an enlarged view of area 17 of FIG. 15, illustrating thecrack and dendritic growth between electrodes;

FIG. 17A is an enlarged view of area 17A of FIG. 17, illustratingdendritic growth within the crack;

FIG. 18 is a diagrammatic view of an EMI filter assembly disposed on theoutside of a hermetically sealed can used in medical devices, inaccordance with the present invention;

FIG. 19 is a partially fragmented and enlarged perspective view of leadsextending from the can of FIG. 18;

FIG. 20 is an electrical schematic diagram of the assembly of FIG. 18;

FIG. 21 is a perspective view of a unipolar capacitor comprised ofnon-migratable materials in accordance with the present invention;

FIG. 22 is a cross-sectional view taken generally along line 22—22 ofFIG. 21;

FIG. 23 is a cross-sectional view of the capacitor of FIGS. 21 and 22along the line 23—23 in FIG. 22, illustrating the layout of conductiveground electrode plates therein;

FIG. 24 is a cross-sectional view taken along the line 24—24 in FIG. 22,illustrating the layout of active electrode plates therein;

FIG. 25 is an isometric view of the capacitor of FIG. 21, having a leadwire extending therethrough and attached to a hermetic terminal of ahuman implantable electronic device;

FIG. 26 is an electrical schematic diagram of the assembly of FIG. 25;

FIG. 27 is a cross-sectional view taken generally along line 27—27 ofFIG. 25, illustrating internal components thereof and the use ofnon-migratable materials;

FIG. 28 is an isometric view of a quadpolar surface mounted capacitor inaccordance with the present invention;

FIG. 29 is an electrical schematic diagram of the capacitors of FIG. 28;

FIG. 30 is a cross-sectional view taken generally along line 30—30 ofFIG. 28, illustrating internal components thereof and the use ofnon-migratable materials;

FIG. 31 is an isometric view of an inline quadpolar capacitor assemblyembodying the present invention and incorporating non-migratablematerials;

FIG. 32 is an electrical schematic diagram of the capacitor assembly ofFIG. 31;

FIG. 33 is a cross-sectional view taken generally along line 33—33 ofFIG. 31, illustrating internal components thereof;

FIG. 34 is a cross-sectional view similar to FIG. 1, but illustratinguse of non-migratable materials in accordance with the present inventionand thus having a crack between electrodes thereof without dendriticgrowth;

FIG. 35 is a cross-sectional view of another feedthrough capacitorhaving a crack therein and without dendritic growth due to the use ofnon-migratable materials in accordance with the present invention;

FIG. 36 is a perspective view of an internally grounded biopolarcapacitor layered with glass in accordance with the present invention;

FIG. 37 is a perspective view of a hermetic terminal, having terminalpins or lead wires extending therethrough;

FIG. 38 is an electrical schematic diagram of the capacitor of FIG. 36;

FIG. 39 is a cross-sectional view illustrating the configuration ofactive electrode plates in the capacitor of FIG. 36;

FIG. 40 is a cross-sectional view illustrating the configuration ofground electrode plates in the capacitor of FIG. 36;

FIG. 41 is a perspective view of the capacitor of FIG. 36 attached tothe hermetic terminal of FIG. 37;

FIG. 42 is a cross-sectional view taken generally along line 42—42 ofFIG. 41, illustrating internal components of the assembly, and a glasslayer thereon;

FIG. 43 is a cross-sectional view of a feedthrough capacitor assembly,wherein the inner metallization of the capacitor has been removed inaccordance with the present invention;

FIG. 44 is a cross-sectional view of another feedthrough filterassembly, wherein the inner and outer metallization of the capacitor hasbeen eliminated in accordance with the present invention;

FIG. 45 is a cross-sectional view of a feedthrough filter capacitorassembly, wherein a terminal pin directly contacts the active electrodeportions of the capacitor in accordance with the present invention;

FIG. 46 is an enlarged sectional view taken generally from area “46” ofFIG. 45, illustrating a knurled or roughened portion of the terminalpin;

FIG. 47 is a cross-sectional view of an integrated feedthrough capacitorassembly embodying the present invention; and

FIG. 48 is a partially fragmented exploded perspective view of amulti-lead feedthrough showing an insulating sheet between thefeedthrough and filter support assembly and incorporating chipcapacitors in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the accompanying drawings for purposes of illustration, thepresent invention resides in EMI filter assemblies which are adapted fordirect body fluid exposure without the formation of harmful metalmigration, dendritic growth, or whiskers.

With reference to FIGS. 18–20, a metal can or housing 100 is illustratedwhich is exemplary of those used to enclose the electronics of animplantable medical device, such as a pacemaker or the like. Theseelectronics typically include a battery, a circuit board, integratedcircuits, and a variety of other components and sensors (not shown). Inthe prior art, as described above, the capacitor is disposed within thehousing 100, which is closed, such as by welding, and hermeticallysealed to protect all of the interior electronic components from theintrusion of body fluids or other contaminants. Terminal pins or leadwires 102 extend into the housing 100 in insulative relationship withthe housing and pass through an EMI feedthrough filter capacitor, as iswell-known in the art.

However, using the present invention, the capacitor 104 can be disposedoutside of the housing 100, advantageously saving space on the inside ofthe unit housing 100. In order to accomplish this, the feedthroughcapacitor 104 and the lead wires 102 are all formed of non-migratablematerials, such as noble metals, which do not migrate in the presence ofelectrical bias and body fluid.

As described above, it takes three ingredients to form a dendrite or toet up the conditions for metal migration. Previously, the ingredientthat was removed is the moisture or thin film by inserting the capacitor104 within the housing 100 which is hermetically sealed. Thus,migratable materials such as in, or silver along with a bias voltagecould be present without harm. However, in the presence of body fluid,or other moisture, any migratable material present can and will lead tothe formation of metal migration, dendrites and the like. Thus, it is aprimary feature of the present invention that the capacitor is designedto operate in the presence of moisture or moisture films by utilizingnon-migratable materials, such as noble metals and alloys, that cannotmigrate.

In FIG. 19, an enlarged view of the lead wires 102 extending through thehousing 100 is shown. Each of the lead wires 102 is in insulativerelationship with the housing 100 by insulating material 106, which isshown for illustrative purposes. It will be obvious to one skilled inthe art that there are a variety of methodologies that can be used tomaintain the four lead wires 102 in non-insulative relationship with themetallic can or housing 100. These include the use of aluminainsulators, glass seals, or a ferrule or individual unipolar ferruleswith gold-brazed alumina insulators. The center ground pin 108 may behollow, as illustrated, whereby after hermetic sealing by laser weldingof the lid of the housing (not shown) one could then use the hollowground pin 108 to pull a vacuum and then back fill the inside with drynitrogen. While still in a nitrogen tank, hollow tube 108 would then behermetically sealed by welding, ball insertion or the like. Thecapacitor illustrated in FIG. 18 is of the internally grounded type,which simplifies the assembly. However, it will be appreciated by thoseskilled in the art that it is not necessary that the capacitor 104 be ofinternally grounded construction, instead surface mounted technologysuch as that described by U.S. Pat. No. 5,333,095 or other capacitortypes so long as it is constructed of materials in accordance with thepresent invention so that the capacitor can be disposed in directcontact with body fluids outside of the housing 100 of the medicaldevice.

FIG. 21 illustrates a monolithic feedthrough ceramic capacitor 110having metallization of non-migratable materials on an inner terminalsurface 112 and an outer terminal surface 114. The capacitor 110 may beconstructed using conventional manufacturing methods, in terms ofsilkscreen, punching, hole drilling, and the like. However, themetallization materials for the inner and outer termination surfaces 112and 114 are typically comprised of a noble metal or alloy, such as puregold, pure platinum, or the like. Other metals include titanium,platinum/iridium alloys, tantalum, niobium, zirconium, hafnium, nitinol,stainless steel and Co—Cr—Ni alloys such as MP35N, Havar® and Elgiloy®.See Table 1 below. These metals are biocompatible and are also known tonot migrate or form dendrites in the presence of moisture or body fluidsolutions. It should be understood that the termination surfaces 112 and114 can be coated with the noble metals or alloys which arenon-migratable, or comprised entirely of such metals. The importantaspect is that those portions that are potentially in contact withmoisture be of non-migratable material to prevent dendritic growth.

TABLE 1 LIST OF CONDUCTIVE, ATTACHABLE, NON-MIGRATING BIOCOMPATIBLEMATERIALS Conductive Metal Metals and Alloys Compounds Polymers*Titanium ZrC Polyethylene Oxide with ionic addition such as NaCl (seeU.S. Pat. No. 6,295,474) Platinum and ZrN Polyurethane platinum/iridiumallows Tantalum TiN Silicone Niobium NbO Polyesters Zirconium (oftenused in TiC Polycarbonate knee joint replacements) Hafnium TaCPolyethylene Nitinol Polyvinyl Chloride Co—Cr—Ni alloys such asPolypropylene MP35N, Havar ®, Elgiloy ® Stainless Steel MathylacrylateGold (has been used as a Para-xylylene stent coating) PolypyrrholEpoxies Polyimides *Any of the commonly used implantable polymersmentioned above can be made conductive by adding a particulate fillersuch as Pt powder. Others: Pyrolytic carbon and Tra-Duct 2902 conductiveadhesive.

FIG. 22 is a cross-section of the novel unipolar feedthrough capacitorof FIG. 21.

Within the capacitor 110 are active 116 and ground 118 electrodes. Ascracks can form in the non-conductive or dielectric material 120 fillingthe capacitor 110 and separating the electrodes 116 and 118, thuspossibly leading to dendritic growth between the electrodes 116 and 118,in a particularly preferred embodiment of the present invention theelectrodes 116 and 118 are also formed of non-migratable material.Preferably, these electrodes 116 and 118 are constructed of platinum ora platinum alloy. The use of platinum electrodes 116 and 118 enables thecapacitor to be a high fire capacitor. That is, the capacitor would haveto be sintered at a much higher temperature than is typically used inthe industry. However, there are other materials which could be usedwhich would form suitable alloys that would not tend to migrate. Forexample, a ternary system, comprised of an alloy of gold, platinumpalladium, could be used. The use of ternary electrodes is known,however, never in combination with the other material described hereinand never used in a human implant application where direct body fluidexposure would be expected.

With reference now to FIGS. 25–27, the unipolar capacitor 110 isinstalled to a hermetic terminal 122, such as a ferrule, of a humanimplantable electronic device. As is well-known in the art, a lead wireor terminal pin 124 extends through the capacitor 110, the purpose ofthe capacitor 110 being to prevent EMI from disturbing or interruptingthe function of internal circuitry and components of the implantablemedical device. The hermetic terminal, or ferrule, is typicallycomprised of titanium or the like. While not of a noble metal or alloythereof, titanium is biocompatible and is not migratable. The lead wire124 should be coated with or solidly formed of a non-migratablematerial, typically gold or platinum or its alloys.

The electrical connecting material 126 between the lead wire 124 and theinner termination surface 112 must also be made of material that cannotmigrate or form dendrites. Similarly, the electrical connection 128between the outer termination surface 114 and the ferrule 122 must alsobe comprised of a bio-stable, non-migratable material. Such electricalconnections 126 and 128 are typically comprised of thermal-settingconductive adhesives, welding or soldering materials. In a preferredembodiment, such thermal-setting conductive epoxies or polyimides wouldcomprise gold or other biocompatible metals as a conductive filler. Saidfillers may be in powder form. Other suitable filler materials includeplatinum, or platinum coated spheres of niobium or tantalum. As thecapacitor's outer surface metallization is typically of pure gold orpure gold plating, the connection between the pure gold and the titaniumis typically done with a gold-filled conductive polyimide. In a similarmanner, the connection between the lead wire 124 which may be comprisedof platinum iridium and the capacitor's inside diameter 112 which istypically comprised of gold, is preferably a gold-filled conductivepolyimide. Alternative materials, such as gold brazing compound or thelike, which forms a non-migratable material, may also be used. Such goldbrazed material is pure gold, pure platinum, or equivalent such that itis noble and therefore does not migrate. A special platinum or goldbearing fired on glass frit may also be used. See Table 1 above.

With respect to the conductive connections, a conductive polyimide ispreferable to a conductive epoxy because of its generally highertemperature rating, although either may be used. An alternative to theuse of a conductive thermal-setting adhesive would be the use of anon-migratable weld, braze or solder compound, such as pure gold brazeor weld.

With particular reference to FIG. 27, an insulator 130 is disposedbetween the ferrule 122 and the lead wire 124. The interface between theinsulator 130 and the terminal pin 124 and ferrule 122 must behermetically sealed. Such seal 132 is comprised of a suitablebiocompatible material such as pure gold braze or TiCuSiI or CuSiI. Itshould be noted that TiCuSiI and CuSiI are brazing alloys that bind upthe silver so tightly that it cannot readily migrate and form adendrite.

Thus, the capacitor 110 and assembly illustrated in FIGS. 21–27 iscomprised of materials that have been constructed of suitablenon-migratable materials, such as noble metals, such that even in thepresence of body fluid and bias voltage would not migrate and form adendrite. That is, the capacitor inside and outside termination surfacediameters 112 and 114, the active and ground electrodes 116 and 118, thelead wire 124, and connective materials 126, 128 and 132 are allnon-migratable material, at least where exposed to the body fluid. Thus,dendritic growth is prevented.

With reference now to FIGS. 28–30, the present invention is not limitedin the type or configuration of capacitor used. A quadpolar surfacemounted feedthrough capacitor 134 is illustrated in these figures and,as discussed above, is entirely constructed of non-migratable materials,at least where exposed to body fluids and moisture. Thus, all of thematerials exposed to the body fluid side are made of materials that willnot form dendrites or migrate. That is, the portion of the lead wire124, inner surface metallization 112, connective material 126, outertermination surface 114, and connective material 128 to the ferrule 122which are exposed to the body fluid are comprised of such non-migratablematerials as described above. Preferably, the electrodes 116 and 118 aswell as the seals 132 of the insulator 130 are also comprised ofnon-migratable materials as described above.

As mentioned above, the lead wires 124 and inner and outer metallization112 and 114 of the capacitor can be plated, such as depositingelectroplated nickel first then overplating with pure gold or the likeplating, or otherwise coated with the non-migratable material.Alternatively, they are comprised of such materials.

With reference now to FIGS. 31–33, an in-line quadpolar capacitor 136 isshown surface mounted on the body fluid side of the hermetic terminal122 of a human implantable medical device. The lead wires or terminalpins 124 are coated with or formed of a non-migratable material, such asa noble metal including gold or platinum. Similarly, as described above,the connection 126 between the inner termination surface 112 and thelead 124 is a non-migratable material, such as those described in Table1 above. In this embodiment, a novel hermetic terminal with gold bondpads 138 is used. A non-migratable conductive connector 128, such asgold or platinum filled thermal-setting conductive polyimide or puregold braze or the like is used to connect the outer termination surface114 to the gold bond pad 138. Thus, on the body fluid side, thoseportions of conductive components which are exposed to the body fluidare comprised of non-migratable materials to prevent the formation ofdendrites.

With reference to FIGS. 34 and 35, capacitor assemblies 140 and 142 areillustrated comprised of the non-migratable materials as discussedabove, and having a sealant 144, such as the epoxy sealant disclosed inU.S. Pat. No. 6,055,455, the contents of which are incorporated byreference herein. In this case, the capacitors 140 and 142 aredeliberately shown with fractures or cracks 146, resulting during themanufacturing process as described above. However, it will be noted thatthese cracks have no metal migration or dendrite within them. This isdue to the fact that the capacitors 140 and 142 are entirely constructedof materials that do not migrate. This includes the lead wire terminalpin 124, inner and outer termination surfaces 112 and 114, electrodes116 and 118, and connective materials 126 and 128. Thus, even a fairlylarge crack in the capacitor 140 or 142 does not present a long-termreliability problem, particularly for a low-voltage device. This is dueto the fact that no harmful metal migration in the form of dendrites ispossible.

Accordingly, in both capacitor structures 140 and 142, the penetrationthrough the non-conductive sealing epoxy 144, shown on the top of thecapacitor, is not a problem. Even though it is expected that over a longperiod of time body fluid would penetrate through the covering epoxy 144through bulk permeability or through micro-separations due to lack ofadhesions, the capacitor and its interconnections have been allconstructed in accordance with the present invention so as not to becomprised of migratable materials, thus preventing the formation ofinsulation resistance reducing dendrites.

FIGS. 36–42 show an internally grounded bipolar capacitor 148 embodyingthe present invention. The capacitor 148 is designed to be surfacemounted to a hermetic terminal 122, such as the illustrated ferrule.Capacitor 148 has been specially prepared during manufacturing to laydown a very thin layer of glass 150 on at least its top surface, andpreferably both its top and bottom surfaces, as illustrated. It has beenfound that such glass layers 150 not only render the overall capacitor148 stronger that it will better resist both mechanical and thermalstress during handling, installation and assembly of the implantablemedical device, but also optimizes the capacitors 148 resistance tomoisture or penetration by fluids. Thus, unlike the aforementioned epoxysealants, the use of a glass sealant may provide sufficient resistanceto penetration by fluids so as to enable a capacitor of migratablematerials to be placed on the body fluid side. However, in aparticularly preferred embodiment, the conductive components which mightbe exposed to the body fluid are comprised of non-migratable material toensure that the harmful metal migration and dendrite formation discussedabove will not occur.

Applying the glass layer 150 can be done in various manners. Forexample, after capacitor firing, the capacitor can be run through aglass-sealing kiln very near the melting point of the glass, but justbelow it. The capacitor 148 would be placed on glass sheets which wouldbe run through the furnace allowing some of the glass to diffuse intothe surfaces of the ceramic capacitor 148. The glass has the effect ofreducing some of the capacitor porosity and filling it with insulatingglass. This cuts down on any tendency to form a dendrite and also makesthe capacitor itself more moisture resistant. Another technique ofapplying the glass layer 150 would be the use of a fine glass or groundpowdered glass, or a paste like frit which would be applied to thecapacitor which is then run through a firing furnace. High volumeapplications could be by silk-screening or spray processes After themanufacturing of the ceramic capacitor 148 at very high temperature, itwould be possible to lay down a very thin glass layer, which would befired or co-fired into place.

The capacitor 148 of FIG. 36 is designed to be mounted to a hermeticterminal, such as the ferrule 122 illustrated in FIG. 37. This ferrulehas been simplified into a rectangular shape, although in actualpractice it can take have many flanges and take on many shapes andconfigurations depending upon its application. The two outer most leads124 are formed in insulative relationship with conductive ferrule 122 byuse of insulating connectors or sealants 152. The center lead wire, orground wire 154 is grounded and directly brazed or welded to theconductive ferrule 122 using non-migratable material 128, such a goldbraze or the like. The grounded pin 154, which is brazed or welded tothe ferrule 122 with a non-migratable material 128 is the connectionpoint for the capacitor's internal ground electrode plates 118.

FIGS. 39 and 40 illustrate the active and ground electrode plates usedin such a configuration, such as that described in U.S. Pat. No.5,905,627. With reference now to FIGS. 41 and 42, as discussed above,preferably the capacitor 148 and assembly are manufactured in accordancewith the teachings of the present invention. That is, the internalelectrode plates 116 and 118 are comprised of platinum or alloys ofgold, platinum or palladium. Other electrode compositions that would notmigrate in the presence of body fluids are also acceptable. Moreover,the conductive connections 126 and 128 are comprised of non-migratablethermosetting or brazing material such as that described above. Thehermetic seal mechanical connections 132 are preferably comprised of agold braze of the like. An alternative to this would be to use a glassseal where a compression or a fusion seal is formed between the ferruleand the outer lead wires 124 such that no metal joining is required atall.

One advantage of the internally grounded capacitor 148 is that it doesnot require any perimeter or outer termination metallization at all.Neither does it require any electrical or mechanical connection betweenthe capacitor 148 and the metallic ferrule 122 as this connection occursbetween the ground terminal pin 154 and the ferrule 122. As there is nocapacitor outer metallization, the connectors 128 between the groundlead wire 154 and the ferrule 122 are comprised of non-migratablematerials, as are the seals and connectors 132 between the insulators130 and the ferrule 122. Of course, the connective material 126 betweenin the inner diameter termination surfaces 112 and the lead wires 124are comprised of non-migratable materials as well.

With reference now to FIG. 43, all prior art monolithic ceramiccapacitors have been constructed with termination materials. Suchtermination materials cover both rectangular MLC chip capacitors andfeedthrough chip capacitors with one or more passageways. The reasonsfor such termination metallization materials are: (1) to provideelectrical connection to the active and ground electrode plates, whichare set in parallel; and (2) to provide a surface wherein one can solderor otherwise make conductive attachments from the capacitor to othercomponents in the circuitry. In the specific case of a human implantdevice as illustrated and described above, termination metallizationmaterials are utilized in the connection from the capacitor activeelectrode plates and terminal pin or lead wire, and the connectionbetween the capacitor ground electrode plates and the metallic ferrule.

The assembly 156 illustrated in FIG. 43 is similar to that illustratedand described above in FIGS. 25–27, except that the feedthroughcapacitor 158 does not include an inner termination metallizationsurface, shown by the reference number 112 in FIGS. 25–27. Instead, thelead wire or terminal pin 124 is conductively coupled to the set ofactive electrodes 116 of the capacitor 158 solely with the electricalconnective material 126. As discussed above, the connective material 126is non-migratable and can be comprised of such materials as gold orplatinum-filled thermal-setting conductive polyimide or any otherconductive material that has been loaded with suitable particles such asgold or platinum such that it can make a direct electrical contact withthe one or more electrodes 124 and be biocompatible.

It is important that the conductive thermal-setting material 126penetrate all the way down through the one or more passageways of thefeedthrough capacitor 158. This is best accomplished by injection orcentrifuging. Accordingly, it is important that this material 126 not beallowed to extend underneath the capacitor 158 such that it could causea short between the ferrule 122 or the outer metallization 114, which isstill present in the embodiment illustrated in FIG. 43. Accordingly, aninsulating material or insulating washer 160 is disposed below thecapacitor 158 to prevent material 126 from migrating or penetrating intoareas where it would be undesirable. In a particularly preferredembodiment, the insulating material 160 is an adhesively coatedpolyimide washer.

Of course, as discussed above, the one or more electrodes 124 would alsobe of non-migratable material such as a noble metal including platinumor gold or an alternative alloy consisting of gold platinum andpalladium. The thermal-setting conductive material 128 used toelectrically connect the conductive ferrule 122 with the outermetallization 114 of the capacitor 158 is comprised of non-migratablematerials as described above.

Whereas the present invention is primarily directed to human implanteddevices and applications, the embodiments illustrated in FIG. 43 havemuch broader application for all feedthrough capacitors whether they befor medical implant or not. The concept of making electrical connectionfrom a lead wire or to the outside diameter without the need fortermination material has obvious advantages to those skilled in the art.It is very labor-intensive to apply these termination materials, whichinvolve several process and termination firing steps. Eliminating theinner termination surface 112 and electrically coupling the lead wire124 directly to the active electrode plates 116 with material 126eliminates a number of process steps relating to prior art capacitorinside diameter termination material.

With reference now to FIG. 44, a feedthrough capacitor assembly 162 isillustrated which is similar to that illustrated in FIG. 35. However,the feedthrough capacitor 164 embedded within the surrounding metallicferrule 122 does not include inside diameter or outside diametermetallization (labeled with reference numbers 112 and 114 in FIG. 35).Instead, the one or more feedthrough holes, which may be of anygeometry, are filled with the conductive material 126, as describedabove in relation to FIG. 43. Conductive material 128, which maycomprise the same material as 126, directly conductively couples thesecond set of ground electrode plates 118 to the hermetic terminalferrule 122. Once again, insulative material, typically in the form of awasher 160, prevents shorting of the capacitor.

With reference now to FIG. 45, yet another assembly 164 is illustratedwhich shows an alternative method of electrically coupling a terminalpin or electrical lead 166 to the internal electrode set 116 of thecapacitor 168. In this case, the pin or wire 166 is designed to form avery tight or pressed fit within the inside diameter or passageway ofthe capacitor 168. In the instance of an inner metallization material112, a mechanical connection is made between the lead wire 166 and thecapacitor metallization 112. As illustrated in FIG. 45, the innermetallization 112 may be absent such that the active electrode plates116 directly contact the terminal pin or electrical lead wire 166 eitherthrough the enlargement of the terminal pin 166 or the reduction indiameter of the passageway through the capacitor 168. In a particularlypreferred embodiment, the electrical lead 166 has been prepared prior toinserting with a knurled, sputtered or roughened area 170 whichcoincides with the internal electrode set 116 to increase the electricalcontact surface area to either the capacitor metallization 112 ordirectly to electrodes 116.

It will be appreciated by those skilled in the art that the embodimentsillustrated in FIGS. 43–46 incorporate the non-migratable materialspreviously discussed so as to have application in implantable biomedicaldevices in which the components of the EMI filter assembly, includingthe capacitor, are exposed to body fluid. The selection and use of thenon-migratable materials and the construction of the capacitor, terminalpin or lead wire, and conductive connections provide a biocompatiblesurface which prevents dendritic growth and the like.

With reference now to FIG. 47, an integrated feedthrough capacitor 172is illustrated, as previously described in U.S. Pat. No. 6,008,980, thecontents of which are incorporated herein. A novel feature of thispatent is that the feedthrough capacitor 172 itself becomes its ownhermetic seal. This is desirable as it eliminates a number of componentsand process steps. Incorporating the use of non-migratable materials inaccordance with the present invention and as described above allows thecapacitor 172 to be placed on the body fluid side of the hermetic seal.

In the previously illustrated and described embodiments, the capacitorshave been feedthrough capacitors. However, it will be appreciated bythose skilled in the art that the present invention is not limited tosuch feedthrough capacitors.

With reference now to FIG. 48, an EMI filter assembly 172 is shown whichis similar to that illustrated and described in U.S. Pat. No. 5,896,267,the contents of which are incorporated by reference herein. The assembly172 includes multiple leads 174 extending through feedthrough 176 andinsulating film 178. A capacitor support assembly 180 supports two chipcapacitors 182 and 184. Insulating film 178 is disposed between themulti-lead feedthrough 176 in the capacitor support assembly 180. Ofcourse, the arrangement is applicable to feedthrough assemblies of anynumber of leads. Insulating film 178 is shaped to contour match that ofthe substrate 186. The leads 174 extend through lined apertures 188 and190 of the insulating film 178 and substrate 186. As such, the leads 174do not extend through the chip capacitors 182 and 184, but the chipcapacitors 182 and 184 are designed so as to present active and groundelectrode surfaces which interact with the leads 174 and groundingterminal, as described in U.S. Pat. No. 5,896,267. The capacitors 182and 184 and the pertinent conductive connection materials, leads 174,etc. are coated or comprised of non-migratable materials as discussedabove so as to be placed on the body fluid side of the applicableimplantable device outside of the hermetic terminal.

Thus, it will be appreciated by those skilled in the art thatmanufacturing feedthrough filter capacitor assemblies in accordance withthe teachings of the present invention, namely, the use ofnon-migratable material such as noble metals and the like, and/or glasssealing layers, allows the capacitor to be disposed outside of thehousing of the medical device without the formation of harmful dendritesdue to metal migration. The free space within the housing can enable thehousing to be smaller, incorporate a larger battery, more sophisticatedelectronics. Other benefits will be appreciated by those skilled in theart.

Although several embodiments of the present invention have beendescribed in detail for purposes of illustration, various modificationsof each may be made without departing from the spirit and scope of theinvention. Accordingly, the invention is not to be limited, except as bythe appended claims.

1. An EMI filter capacitor assembly for an active implantable medicaldevice, comprising: a capacitor directly exposed to body fluid whenimplanted, including first and second sets of electrode platescomprising a non-migratable and biocompatible material; a conductivehermetic terminal comprising a non-migratable and biocompatible materialadjacent to the capacitor and in conductive relation to the second setof electrode plates; and a conductive terminal pin having an outersurface comprising a non-migratable and biocompatible material at leastwhere exposed to body fluid in conductive relation with the first set ofelectrode plates, the terminal pin extending through the hermeticterminal in non-conductive relation.
 2. The assembly of claim 1, whereinthe first and second sets of electrode plates comprise a noble metal ornoble metal composition.
 3. The assembly of claim 2, wherein the firstand second sets of electrode plates comprise gold, platinum, agold-based alloy, or a platinum-based alloy.
 4. The assembly of claim 1,wherein the outer surface of the terminal pin comprises a noble metal ora noble metal composition.
 5. The assembly of claim 4, wherein the outersurface of the terminal pin comprises gold, platinum, titanium, niobium,tantalum, a gold-based alloy, or a platinum-based alloy.
 6. The assemblyof claim 1, wherein the outer surface of the terminal pin directlycontacts the first set of electrode plates.
 7. The assembly of claim 1,wherein the capacitor includes a first termination surface comprising anon-migratable and biocompatible material conductively coupled to thefirst set of electrode plates and the terminal pin.
 8. The assembly ofclaim 7, wherein the first termination surface comprises a noble metalor a noble metal composition.
 9. The assembly of claim 8, wherein thefirst termination surface comprises gold, platinum, a gold-based alloy,or a platinum-based alloy.
 10. The assembly of claim 7, wherein theterminal pin directly contacts with the first termination surface. 11.The assembly of claim 7, including a connection material comprising anon-migratable and biocompatible material conductively coupling theterminal pin and the first termination surface.
 12. The assembly ofclaim 11, wherein the connection material comprises a brazing, weldingor soldering material.
 13. The assembly of claim 12, wherein thebrazing, welding or soldering material is selected from the groupconsisting of: titanium, platinum and platinum/iridium alloys, tantalum,niobium, zirconium, hafnium, nitinol, Co—Cr—Ni alloys, stainless steel,gold, ZrC, ZrN, TiN, NbO, TiC, TaC, gold-bearing glass frit, TiCuSiI,CuSiI, and gold-based braze.
 14. The assembly of claim 11, wherein theconnection material comprises a conductive thermal-setting material. 15.The assembly of claim 14, wherein the thermal-setting material comprisesa polymer selected from the group consisting of: epoxies, polyimides,polyethylene oxide, polyurethane, silicone, polyesters, polycarbonate,polyethylene, polyvinyl chloride, polypropylene, methylacrylate,para-xylylene, and polypyrrhol.
 16. The assembly of claim 15, whereinthe thermal-setting material includes a non-migratable and biocompatibleconductive filler.
 17. The assembly of claim 16, wherein the conductivefiller is selected from the group consisting of titanium, platinum andplatinum/iridium alloys, tantalum, niobium, zirconium, hafnium, nitinol,Co—Cr—Ni alloys, stainless steel, gold, ZrC, ZrN, TiN, NbO, TiC, TaC.18. The assembly of claim 1, wherein the second set of electrode platesdirectly contacts the hermetic terminal.
 19. The assembly of claim 1,including a connection material comprising a non-migratable andbiocompatible material conductively coupling the second set of electrodeplates and the hermetic terminal.
 20. The assembly of claim 1, whereinthe capacitor includes a second termination surface comprising anon-migratable and biocompatible material conductively coupled to thesecond set of electrode plates and the hermetic terminal.
 21. Theassembly of claim 20, wherein the second termination surface comprises anoble metal or a noble metal composition.
 22. The assembly of claim 21,wherein the second termination surface comprises gold, platinum, agold-based alloy, or a platinum-based alloy.
 23. The assembly of claim20, including a connection material comprising a non-migratable andbiocompatible material conductively coupling the second terminationsurface and the hermetic terminal.
 24. The assembly of claim 23, whereinthe connection material comprises a brazing, welding or solderingmaterial.
 25. The assembly of claim 24, wherein the brazing, welding orsoldering material is selected from the group consisting of: titanium,platinum and platinum/iridium alloys, tantalum, niobium, zirconium,hafnium, nitinol, Co—Cr—Ni alloys, stainless steel, gold, ZrC, ZrN, TiN,NbO, TiC, TaC, gold-bearing glass frit, TiCuSiI, CuSiI, and gold-basedbraze.
 26. The assembly of claim 23, wherein the connection materialcomprises a conductive thermal-setting material.
 27. The assembly ofclaim 26, wherein the thermal-setting material comprises a polymerselected from the group consisting of: epoxies, polyimides, polyethyleneoxide, polyurethane, silicone, polyesters, polycarbonate, polyethylene,polyvinyl chloride, polypropylene, methylacrylate, para-xylylene, andpolypyrrhol.
 28. The assembly of claim 27, wherein the thermal-settingmaterial includes a non-migratable and biocompatible conductive filler.29. The assembly of claim 28, wherein the conductive filler is selectedfrom the group consisting of titanium, platinum and platinum/iridiumalloys, tantalum, niobium, zirconium, hafnium, nitinol, Co—Cr—Ni alloys,stainless steel, gold, ZrC, ZrN, TiN, NbO, TiC, TaC.
 30. The assembly ofclaim 1, wherein the hermetic terminal comprises a material selectedfrom titanium, platinum and platinum/iridium alloys, tantalum, niobium,zirconium, hafnium, nitinol, Co—Cr—Ni alloys, stainless steel, gold,ZrC, ZrN, TiN, NbO, TiC, and TaC.
 31. The assembly of claim 1, includinga biocompatible hermetic insulator disposed between the hermeticterminal and the terminal pin, and seal joints connecting the insulatorand the hermetic terminal, the seal joints comprising a non-migratableand biocompatible material.
 32. The assembly of claim 31, wherein theseal joints are selected from the group consisting of: titanium,platinum and platinum/iridium alloys, tantalum, niobium, zirconium,hafnium, nitinol, Co—Cr—Ni alloys, stainless steel, gold, ZrC, ZrN, TiN,NbO, TiC, TaC, gold-bearing glass frit, TiCuSiI, CuSiI, gold-basedbraze, and polymers including epoxies, polyimides, polyethylene oxide,polyurethane, silicone, polyesters, polycarbonate, polyethylene,polyvinyl chloride, polypropylene, methylacrylate, para-xylylene, andpolypyrrhol.
 33. The assembly of claim 1, including a glass layercovering a top surface of the capacitor.
 34. The assembly of claim 33,including a glass layer further covering a bottom surface of thecapacitor.
 35. The assembly of claim 1, wherein the capacitor comprisesa chip capacitor.
 36. The assembly of claim 1, wherein the capacitorcomprises a feedthrough capacitor having a passageway for permitting theterminal pin to extend therethrough.
 37. The assembly of claim 36,wherein the capacitor includes multiple passageways for reception ofmultiple terminal pins therethrough, each terminal pin having an outersurface comprising a non-migratable and biocompatible material at leastwhere exposed to body fluid.
 38. The assembly of claim 1, wherein theactive implantable medical device is selected from the group consistingof: cardiac pacemakers, cardioverter defibrillators, neurostimulators,internal drug pumps, cochlear implants, and ventricular assist devices.